Transparent conformal polymer antennas for RFID and other wireless communications applications

ABSTRACT

An optically transparent conformal polymer antenna and a method for producing the antenna from optically transparent conductive polymers. The method includes selecting an antenna design; providing an optically transparent conductive polymer material capable of being printed using an ink-jet printer device; and printing layers of the polymer in the desired antenna design pattern onto a substrate. The surface tension of the polymer solution is adjusted to allow the material to pass through a printer head for printing on a flexible substrate. The material is modified to have a higher conductivity than regular conductive polymer materials so that a suitable antenna may be formed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 13/266,303, filed Nov. 22, 2011, which is the National Stage ofInternational Application No. PCT/US2010/032541, filed Apr. 27, 2010,which claims the benefit of U.S. Provisional Application No. 61/173,056,filed Apr. 27, 2009, the disclosures of which are incorporated herein byreference in their entireties.

STATEMENT OF FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contractDAAB07-01-9-L504 awarded by the United States Army C-E LCMC. Thegovernment has certain rights in the invention.

TECHNICAL FIELD

The present invention relates generally to optically transparentconductive polymer antennas for RFID and other wireless communicationsapplications and to methods for fabricating such antennas on a varietyof materials.

BACKGROUND

There is an increasing demand for transparent antennas that can beattached to non-traditional surfaces and materials, such as conformalantennas adhering to flexible fabrics and plastic substrates. Antennasmeeting these needs have previously been prototyped through the use ofconductive inks on inflexible substrates. However, while highlyconductive, these ink antennas are not optically transparent and do notadhere reliably on flexible substrates.

Optical translucence allows for the potential to hide an antenna or tomake the antenna virtually invisible when mounted on a transparentsubstrate, such as a vehicle's windshield. While conductive polymershave lower conductivities in comparison to traditional antenna materialsand conductive inks, they have many advantages such as their low cost,ease of processing, and potential for all-additive ink-jetmanufacturing. Previously reported conductive polymer antennas usedpolymers that included silver particles or were made from polymers thatneeded to be thick in order to achieve high conductivity. However, thesepreviously reported antennas were neither transparent nor flexible.

Fabrication of translucent conformal antennas utilizing additiveprinting methods offers a stark contrast to traditional subtractivefabrication techniques generally used in circuit board and RFproduction, such as milling and chemical etching. As an additiveprocess, printing can achieve patterns and geometries while using aminimum of material, which poses a large economic advantage especiallywhen printing precious metals or other high cost materials. The additivenature of the process also allows for the use of a wide array ofsubstrates including traditional boards such as FR-4 and ceramic-baseddielectric materials. The low-temperature and non-contact nature of theprocess also allow for the use of many non-traditional and less rigidsubstrates such as plastics, polymers, fabrics, and paper.

Radio frequency identification (RFID) is a very popular technology foran increasing number of applications. The most basic passive RFID tagsystem consists of an interrogator (an infrastructure used to querytags) and the tags themselves. The core technology is the backscatteringtechnique that enables very inexpensive circuits without batteries toreturn information to an interrogator. Because of the simplicity of theoptically transparent printed conformal polymer antenna, the RFID tagscan be made smaller and less expensive.

Previous work has focused on creating antennas that provide goodmatching and high efficiency for RFID systems. Examples of these designsinclude meandering dipoles, which reduce the total length of the dipoleantenna by bending the two poles. Non-traditional (silver-ink) materialshave been used before for RFID applications due to their low cost.However, they are not as flexible, conformal, nor as transparent asconductive polymers. Conductive polymers have been used for very lowfrequency applications, but due to their low conductivity have onlyrecently been used for radio frequency (RF) applications. There are nowconductive polymers that have high enough conductivity at RF frequenciesto make them suitable for RFID applications. Methods are desired forusing such conductive polymers to create antennas for RFID and otherwireless communications applications.

SUMMARY

The present invention provides a method for reliably producing anoptically transparent conformal antenna that can be attached to avariety of flexible and inflexible substrates. Such a method comprisesthe steps of selecting an antenna design to be applied to a substrate;providing an optically transparent conductive polymer material to aprinting device, the material having a surface tension that allows theprinting device to eject solution drops having a surface tension between0.028-0.060 N/m, and printing desired pattern layers of the materialonto the substrate according to the antenna design. Ultrasonic vibrationmay be applied to the material to remove gases before the antenna designis printed. Also, dimethyl sulfoxide may be added to the conductivepolymer material to increase conductivity and a surfactant may be addedto the conductive polymer material to lower surface tension. Theprinting step may be optimized by applying multiple layers of thematerially sequentially to allow drops to flow together into a thickertrace.

In an exemplary embodiment of the invention, the optically transparentconformal polymer is attached to a substrate through an inkjet printerutilizing microstrip antenna technology. In particular, the methods ofthe invention are used to create anoptically transparent conductivepolymer antenna comprising a non-traditional substrate such as glass ora flexible substrate such as a flexible plastic, a polymer, a fabric, orpaper, and an antenna printed on the substrate. The antenna is formedfrom a conductive polymer material having a surface tension between0.028-0.060 N/m so as to enable the material to pass through a printhead for printing on the flexible substrate. The spacing between printeddrops is preferably between 5 μm and 254 μm. The antenna may beconnected to a machine-readable identification tag to produce an RFIDunit such that the antenna radiates in response to an interrogationfrequency from an interrogator unit. A power source for the antenna mayalso be provided so that the antenna may be used for transmitting andreceiving. A signal processor may be used so that signal received by theantenna can be produced. In an exemplary embodiment, the conductivepolymer material includes dimethyl sulfoxide and a surfactant and has aconductivity of at least 5×10⁵ S/m. The antenna may also be formed as acenter-fed dipole antenna designed for a resonant frequency of 900 MHz.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the process of designing, printing and testing anantenna in accordance with the invention.

FIG. 2 illustrates the procedure for printing the polymer antenna inaccordance with the invention.

FIG. 3 is a topographical profilometer image that shows the dropletsspacing being too large, resulting in a non-uniform layer.

FIG. 4 is a sample 2 dipole antenna.

FIG. 5 is an example of an RFID meandering dipole antenna and diagram.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

A. Fabrication Techniques

Screen printing and mask-bake deposition were initially used to makeantennas; however, these methods offer very little control over layerthickness and limits printed patterns to simple and fairly large shapes.Inkjet printing, in contrast, offers extensive control over thesefactors but also introduces many new variables into the fabricationprocess. Most of these new variables can be classified into eithersolution formulation and physical property issues or process issues,though there are some that span both categories. All samples werefabricated with a Fuji-Dimatix DMP-2831 materials printer. For thesolution to be compatible with the inkjet print head it must firstpossess certain physical properties. Viscosity is ideally such that thefluid can be manipulated by the electronically actuatedmicroelectromechanical membrane of the DMP-2831 materials printer, inthe range of about 5-25 cPs. The surface tension of the solution is alsoimportant, as it will control the amount of driving force from theprinting membrane needed to forcefully eject a drop out of the printnozzle. Ideal solution surface tension ranges from 0.028-0.033 N/mthough solutions up to 0.060 N/m can be printed with reduced dropvelocities. Volatility plays a key role as fluids that evaporate quicklycan leave behind particles that will clog the print nozzles and renderthem useless or lead to streaky and unreliable printing.

The inventors fabricated antennas out of a conductive polymer, a generalPEDOT-PSS. By specification, this polymer has a conductivity of 300 S/m.The polymer was modified with 10% dimethyl sulfoxide (DMSO) to increasethe conductivity, and 2% surfactant (such as Tween-21) to lower thesurface tension. Through testing, it was discovered that theconductivity of the modified solution is approximately 5×10⁵ S/m, whichis over two magnitudes higher than the stock PEDOT-PSS and much closerto the conductivity of copper (5.9×10⁷ S/m). Solutions withconductivities of at least 5×10⁵ S/m are desired for the applicationsdescribed herein.

While all of these properties affect the fluid while inside the printhead, they are also important once the drop is in free fall and aftertouch down on the substrate. The drop size formed upon ejection from thenozzle, as well as the shape of any ligaments and/or satellite drops, isa function of the solution's physical properties and the drivingwaveform. An extremely volatile liquid can also leave behind residue atthe nozzle opening, causing drops to be deflected at angles other thanthe desired vertical upon ejection. Evaporation on the substrate alsocan play a role in how large each drop spreads after landing, affectinglayer continuity and the necessary drop spacing. Once the drop of fluidhas traversed the free space between print nozzle and substrate, thechoice of substrate will come into play. Every substrate will havedifferent wettabilities, which also change drop spreading, causing thedrop to spread or dewet into a smaller footprint.

Coupled with solution formation and physical property issues, theprinting process itself also introduces many new variables. The spacingbetween printed drops can be set from approximately 5-254 μm. Thisallows for excellent control over drop overlap and layer uniformitywhich leads to conductivity across the PEDOT-PSS conductive polymerlayers. Drop dynamics and varying time between drops or layers must beconsidered. Every pattern printed consists of a different number oftotal printed drops. Also, each change in drop spacing will change thetotal drop count. A different amount of drops will result in a change inthe printing time, affecting drop overlap and layering as a function ofwetting (viscosity, surface tension, and substrate dependent) andvolatility. The eventual goal of maximal conductivity in PEDOT:PSSlayers is achieved when the conductive polymer chains are allowed toorient in solution across the entire plane of the printed layer. Bysetting the spacing of printed drops in close enough proximity to allowthe drops to flow together into one continuous pool rather thanindividual drops, consistently maximal conductivity is ensured. Layerthickness, and therefore conductivity, can be controlled by sequentiallyapplying multiple layers of polymer.

B. Analysis of Conductive Polymer

In order to better understand the effects of the printing technique onthe performance of the conductive polymer, several test samples havebeen printed on a PET (polyethylene terephthalate) substrate andcharacterized. A series of uniformly shaped rectangles were printed,varying in both the drop spacing, and number of print layers. Thedifferent drop spacing distances tested were 10 μm, 25 μm, and 50 μm.The rectangles were fabricated at these drop distances for 1, 2, 5, and10 layers. Once printed, the test samples were observed under an opticalprofilometer to determine the thickness of the samples. Then the sheetresistance was measured using a 4 point probe. The results were comparedto determine the effect on sheet resistance by both the printed filmthickness, and the droplet spacing. They were observed under an opticalprofilometer to observe both the topography of the sample, to confirmthat the samples are uniform, and the step size. It was found in FIG. 2that the thickness varied greatly among samples of the same number oflayers, based on the droplet spacing.

It was also found that the samples fabricated with 50 μm droplet spacinghave little to no effectiveness with respect to conductivity. Thedistance between the drops is too great for them to fuse together or tocreate a single layer. The two layer 50 μm spacing sample measured onthe order of hundreds of k/sq sheet resistance, and the one layer samplecould not return a measurable resistance value at all. FIG. 3 is atopographical profilometer image that shows the droplets spacing beingtoo large, resulting in a non-uniform layer. It was found that as thethickness of the sample increases, the sheet resistance tends todecrease. Also, the test results show that as the droplet spacingdecreases, the sheet resistance also decreases. This occurs because asthe distance between droplets is decreased, more droplets are needed tofill the same pattern area. Thus, there is more conductive material inthe same area, increasing the overall conductivity. The profilometerimages also show that the samples with a more uniform surface have amuch lower sheet resistance. The five layer, 10 μm spacing sample has auniform surface within 60 nm on average. The five layer, 1 micronspacing sample has a much lower uniformity, as can be seen by theprofilometer image of FIG. 3. The tests performed on the inkjet printedconductive polymer samples show that the most conductive thin films tendto result from a smaller drop spacing, and a greater number of layersprinted on top of each other. While there were some results that goagainst this trend, they are a result of the affect the four point probehad on the samples, and do not represent the undamaged sheet resistancevalues of the material.

In an exemplary embodiment of the invention, a materials printer, suchas, but not limited to, a Fuji-Dimatix DMP-2831, is used in the antennafabrication process. In accordance with this method, prepared solutionsof conductive inks and polymers are injected into the printer'scartridges and printed out of a programmable number of the print head'snozzles. Each of these nozzles can be actuated by amicro-electromechanical silicon membrane with a controllable drivingwaveform creating a pressure wave that ejects fluid from the nozzle.Several different cartridges with varying nozzle sizes are available,allowing a degree of control in drop size from a few microns to tens ofmicrons. Drop spacing is also fully controllable from less than 5 μm to254 μm, allowing another degree of freedom in affecting layer thicknessand drop overlap. Such robust control over drop size and spacing allowsfor the printing of complex patterns and geometries within a fewminutes.

FIG. 1 illustrates one embodiment for designing and printing antennas inaccordance with the invention. The first step 101 is the creation andediting of the design. Once the antenna is designed, the correspondingartwork is created in the next step 102 using computer aided design andexported as a file in a format such as the Gerber file format. Anoptional step 103 includes performing electromagnetic simulations on thedesigned antenna to create a profile. Following the test, a decisionmust be made regarding the acceptability of the results 104. If theresults are not acceptable, the antenna is redesigned. On the otherhand, if the results are acceptable, the design is sent via the file toa printer software application where the pattern is created 105. Thematerial printing results are analyzed to determine the proper drop size106. Layer analysis is performed through an optical profilometer for ameasurement of the thickness of the antenna. If a more detailed layeranalysis is required, the antenna can be placed into a scanning electronmicroscope. Conductivity analysis can be performed by both a DCresistance measurement, and an AC 4-point probe analysis. An analysiswith an optical spectrometer is used to determine the transparency ofthe antenna. Through these analyses, the trade-off between greater layerthickness/conductivity and transparency is determined, resulting in anyrequired drop size and spacing alterations to achieve the design goals.The drop spacing is then adjusted to a fraction of the singular dropsize 107, creating drop overlap between neighboring drops, allowing fora thicker trace, resulting in higher conductivity. The antenna isprinted and then optionally inspected with a fiducial camera 108. Theacceptability of the results determine if the drop spacing must befurther reduced and the antenna reprinted or if the return loss andradiation pattern of the antenna will be tested. 109, 110, 111.

FIG. 2 illustrates the materials printing procedure. A conductivepolymer is selected based on conductivity requirements 201. Dimethylsulfoxide can be added to the polymer to increase the conductivity ifnecessary. It is then determined if the solution conforms to the printerhead parameters 202. If the solution does not conform, a surfactant isadded to decrease surface tension and aid in printer-head compatibility203. The solution is then passed through a 2 μm filter 204. The solutionis then injected into a print cartridge reservoir 205. Ultrasonicvibration is applied to the solution to remove gases and promote printhead wetting 206. A drop spacing pattern is then printed (5 μm to 254 μmdrop spacing grid) 207. The size of the individual drops is measured anda fraction of the size measured is applied to the drop spacing 208,creating drop overlap between neighboring drops, allowing for a thickertrace, resulting in higher conductivity. The desired pattern is thenprinted 209. If the drop profile is acceptable (layer, thickness, etc.)then the sample is moved to the vacuum oven for bake-off/curing andcross-linking 210, 211, 212. If the drop profile is not acceptable, thespacing between the drops is decreased and new drops are printed 213.

The printed antennas make use of microstrip antenna technology. Theprinted antennas can be realized by using conductors with differentsizes and shapes. The printed conformal polymer antennas that are thetopic of this invention are unique at least in part because theconductors have controllable levels of optical transparency, can befabricated on non-traditional substrates (i.e., glass and fabrics suchas neoprene, vinyl, polyester, and Kevlar), and use materials that aredeposited in an additive manner (i.e., rather than being mechanically orchemically etched).

Specific performance measures and how they are effected by the materialand printing process include frequency, power handling, transmissionrange, form factor, and antenna performance. These factors areelaborated upon below.

Frequency

The microwave frequency range of interest for printed polymer antennasspans the UHF, S, and C bands (roughly 100 MHz-8 GHz). Limitations inregard to feature size possible through printing are irrelevant withregard to the small features, as the shortest wavelengths in questionare several orders of magnitude larger than the current minimum printerdrop spacing of 5 μm. There are no limitations in pattern complexity ordiscontinuity; however, minimum feature size must be above the 5 μmspacing between drops, or allow for drop sizes around 5-100 μm dependingon the solution, substrate, and print head size used. While an increasein thickness of the polymer layer shows higher conductivity, there is acompromise where edge resolution decreases with increasing layerthickness. This is due to a “puddling” effect where the surface tensionof the puddle formed begins to deform the deposited layer from thedesired pattern causing non-uniform evaporation of the solvent.

Power Handling

No power handling maximum has been reached. Screen-printed samples ofmicrostrip transmission lines and both dipole and patch antennas havebeen tested with success up to 5 W which exceeds the power requirementsof most applications of printed polymer antennas.

Transmission Range

There is a tradeoff that can be adjusted, depending on the application,between transparency and conductivity. Both layer thickness as well assolution composition can be controlled to create thicker, moreconductive, and less transparent layers or the opposite in thinner, lessconductive, but more transparent layers. The solution composition can bemodified with the addition of deionized water for a simple dilution,giving more space in the solution between polymer chains and thus ahigher transparency. The composition can also be modified with asurfactant, such as Tween 21, which decreases the solution's surfacetension causing printed patterns to wet or to spread more on asubstrate, effectively spacing the polymer chains out through thismetric. The decreased surface tension also makes the polymer dropletsmore compliant with the inkjet print heads. Additionally, 5-10% dimethylsulfoxide (DMSO) is added to the conductive polymer solution to increaseconductivity. DMSO has a negligible effect on the printing and otherphysical properties of the polymer solution.

Form Factor

The room-temperature printing process and variety of curing processesincluding the relatively low-temperature baking/curing cycle at 70° C.allows for the use of a wide array of substrates such as transparent andconformal polymers and plastics, where traditional metal processing anddeposition is done at much higher temperatures well above the meltingpoint of these materials. More traditional materials such as ceramics,fiberglass, glass, and metals can also be used, as can virtually anysubstrate that fits the size requirements of the printer used to producethe antenna. It is also possible to print on substrates that arenon-uniform in shape or height, including curved structures.

Antenna Performance

All of the aforementioned compromises in thickness, conductivity,transparency, and solution formation are undeniably tied to the finalprinted polymer antenna performance. Each of these can be tuned as aspecific applications mandate. There is also the physical current flowto be considered, specifically with respect to the Skin Effect whichplays a large role in thin film conductors carrying alternating current.The skin effect is the phenomenon in which an alternating current tendsto concentrate in the outer layer of a conductor, caused by theself-induction of the conductor and resulting in increased resistance.At a frequency of 2.4 GHz, calculation would suggest a skin depth on theorder of tens of microns for printed polymer layers. Therefore,extremely thin inkjet-printed antennas on the order of several micronswill not perform at the same levels as those printed at several skindepths, at or above 100 μm. Higher-frequency devices will require asmaller skin depth, and conversely lower-frequency devices will requirelarger skin depth. This effect must be taken into account when designingand printing antennas with a minimal cross-sectional profile.

APPLICATIONS

Examples of Optically Transparent Conductive Polymer Antennas: Dipoleand Meandering Dipole Antennas

2 Dipole Antenna

The initial fabricated conductive polymer (CP) antenna was a simplecenter-fed dipole antenna. The antenna was designed for a resonantfrequency of 900 MHz. Practically speaking the antenna is too large forRFID purposes, which is why a meandering dipole is presented below.However, it provides a good baseline design for future prototypes. Thedimensions of the antenna are 150 mm long and 3 mm wide as seen in FIG.4.

Meandering dipoles reduce the total length of the antenna, which isideal for RFID tags. The inventors chose a design similar to previouslyreported designs to easily match to the highly inductive load of typicalRFID circuits. The resulting antenna, as seen in FIG. 5, is designed for900 MHz center frequency and has physical dimensions listed in Table I.Table 1 lists the dimensions for the meandering dipole antenna usingquantities labeled in FIG. 5. FIG. 5 also demonstrates the flexibilityof the antenna. The meandering design reduces the antenna size by 50%(relative to the 2 antenna), to a total length of 76 mm. The finalsegment of the antenna, seen as D in FIG. 4, can be trimmed in order totune the antenna to the correct resonant frequency. The conductor andground feeds go through the substrate and are connected to the antennatrace with solder and silver paint for the copper and conductive polymerantennas, respectively.

TABLE 1 Variables for the meandering dipole antenna using quantitiesLabeled in FIG. 5 Variable (mm) A B C D L S w Value 4 7 11 8 76 1.5 0.7

One embodiment of an application for optically transparent printedconformal antennas is cell phones and base stations. For cell phones,the antennas could be conformal on the outside of a mobile device,integrated into clothing of a cell phone user, or printed on thewindshield of a vehicle. Antennas for common cellular frequenciesoperating with the full range of transmission powers can be realized.All previously published cellular handset antenna designs can berealized using the proposed technique. However, a key difference is thatsince the antennas can be transparent and printed conformally on amobile device or external structure, space on the internal circuit boardof the mobile device does not need to be used for the antenna(potentially allowing the mobile device to be smaller).

For cellular base stations, rather than using antennas mounted on acellular tower, antennas can be made transparently and integrated intowindows that are high off the ground or on the sides of buildings. Thus,cellular base stations can be constructed that are visually unobtrusive.By logical extension, antenna arrays can also be constructed to providemultidirectional signal coverage.

Another embodiment of optically transparent printed conformal polymerantennas in accordance with the invention includes applications in localand personal area networks making use of Wi-Fi (IEEE 802.11), WiMAX(IEEE 802.16), Bluetooth (IEEE 802.15.1) and/or ZigBee (IEEE 802.15.4).Optically transparent printed conformal polymer antennas could be usedin both nodes in the network and access points (when they are present).Antennas for mobile network devices could be realized in much the sameway as for cellular phones, and all existing network antennaarchitectures can be realized using the printed polymer antennatechnology. With the increasing trend to assign network addresses to amuch greater number of devices (e.g., PDAs, iPods, and home appliances),the ability to unobtrusively deploy an antenna transparently on thepackaging of the device rather than occupy valuable space within thedevice can provide significant improvements in terms of device size andform factor. Access points, when appropriate in the network topology,can be improved in much the same way as the cellular base stationsdiscussed in the previous section.

Another embodiment of optically transparent printed conformal polymerantennas includes applications for satellite communications, includingsatellite radio (XM, Sirius, etc.), Satellite TV (DirectTV, Dish, etc.),satellite data such as internet data, and Global Positioning Satellite(GPS) systems. The antennas and antenna systems are therefore capable ofboth transmit and receive, and have ranges up to and possibly beyondGeosynchronous Earth Orbit at ˜23,000 mi. All published and realizableantennas designs for these systems are expected to be realizable in theprinted antennas. Example implementations include, but are not limitedto, direct print on handheld devices and vehicles, printing on stickerswhich can be affixed to devices, buildings, structures, or vehicles, andlamination within clearcoat, and other types of encapsulation coatings.

Another embodiment of optically transparent printed conformal polymerantennas is radio frequency identification (RFID). The RFID unitincludes a machine-readable identification tag connected to the antenna.The antenna is adapted to radiate in response to an interrogationfrequency from an interrogator unit. The RFID unit may also include apower source for the antenna for transmitting and receiving and a signalprocessor for receiving. A novel way to create antennas of a sizesuitable for RFID tags and that are optically transparent and canconform to different shapes is through the use of conductive polymers.Non-traditional (silver-ink) materials have been used before for RFIDapplications due to their low cost. However, they are not as flexible,conformal, nor as transparent as conductive polymers.

The printed antenna technology of the invention allows for quickly andinexpensively fabricated RFID antennas. With the advantage oftransparent antennas, the RFID applications could include: Inventorytracking (RFID antennas could be printed directly on packaging/clothing,where it would be invisible to the consumer); EZ-Pass-type applications(instead of a plastic box being Velcroed on the windshield, atransparent antenna on a substrate could be stuck onto the carwindshield, eliminating any visual obstructions to be used to for roadtolls and parking); Rescue/Emergency worker locating (allowing emergencyworkers, when going into a dangerous situation, to be fitted with anunobtrusive antenna for location finding); Location finding (where, forexample, an antenna could be directly printed onto a lift pass at a skiresort, and if a skier is lost, tracking could be performed); and publictransportation payment option (public transportation payment optionssuch as bus, rail and subway).

The above listed applications using the printed polymer material havetrade-offs, including transparency vs. read range of the RF antenna.Therefore, each application and their desired performance will have tobe evaluated when fabricating the antenna.

What is claimed:
 1. An optically transparent conductive polymer antennacomprising: a flexible substrate; and an antenna printed on saidflexible substrate, said antenna comprising an optically transparentconductive polymer material not having any electrically conductiveparticles added to the conductive polymer material, said material havinga conductivity that is at least 5×10⁵ S/m and a surface tension between0.028-0.060 N/m so as to enable said material to pass through a printhead for printing on said flexible substrate.
 2. The antenna of claim 1,wherein the antenna is printed on said flexible substrate wherebyspacing between printed drops is between 5 μm and 254 μm.
 3. The antennaof claim 1, wherein the flexible substrate comprises at least one of aflexible plastic, a polymer, a glass, a fabric, and a paper.
 4. Theantenna of claim 1, further comprising a machine-readable identificationtag connected to the antenna to produce an RFID unit.
 5. The antenna ofclaim 1, wherein the antenna is adapted to radiate in response to aninterrogation frequency from an interrogator unit.
 6. The antenna ofclaim 1, further including a power source for the antenna fortransmitting and receiving.
 7. The antenna of claim 1, further includinga signal processor for the antenna for receiving.
 8. The antenna ofclaim 1, wherein the material includes dimethyl sulfoxide and asurfactant.
 9. The antenna of claim 1, wherein the antenna comprises acenter-fed dipole antenna designed for a resonant frequency of 900 MHz.